In rotating machines, such as steam turbines and gas turbines, in order to prevent a working fluid, such as steam, from leaking from a gap formed between a stationary side and a rotational side, contactless seal structures, such as a labyrinth seal, are used (for example, refer to PTL 1). A direct passage type labyrinth seal or a step type labyrinth seal is known as the labyrinth seal.
The direct passage type labyrinth seal is a seal structure that has a shroud provided at the tip of a blade and formed so as to be parallel to an inner peripheral surface of a casing that forms the contour of a rotating machine, and seal fins extending from the inner peripheral surface of the casing toward the blade, and that has a structure as seen through from an upstream side to a downstream side.
The step type labyrinth seal, as disclosed by PTL 1, is a seal structure having seal fins extending from an inner peripheral surface of a casing toward a blade, and a step-shaped shroud provided at the tip of the blade.
In a case where the numbers of seal fins and the dimensions of a gap (fin clearance) between the tip of each seal fin and an outer peripheral surface of the shroud are the same, it is said that the sealing performance of the step type labyrinth seal is higher than the linear type labyrinth seal.